The world's population has a growing need for low-cost, reliable, usable energy, whether that energy is created by burning fossil fuels or by harnessing renewable energy resources, commonly referred to as “clean energy”.
The use of fossil fuels as an energy source is falling out of favour due to growing concerns over the associated environmental impact of greenhouse gas and carbon emissions. Further, there is a concern that fossil fuels are a finite resource that one day will be depleted.
Various systems of harvesting renewable energy have been explored and/or used with varying degrees of success and expense.
Horizontal axis turbines are an example of current methods for harvesting kinetic energy from wind and water flow.
Horizontal axis wind turbines have been used in wind to cause blades to rotate around a central pivot point to generate electricity. Horizontal axis water turbines have been used underwater as a method for harvesting energy in flowing water, such as tidal flows or ocean currents.
Horizontal axis fluid turbines all have limitations and deficiencies. The relative rotational speed of the blades near the pivot point is considerably lower than the relative rotational speed of the blades near the outer tips. The speed of the blades near the tips can be so fast that the blades can be a serious hazard to birds or marine wildlife that come near the turbine. Further, the added torque created by gusts and/or very high fluid flows can cause structural failure.
If all the energy coming from fluid movement through a horizontal axis turbine was extracted as useful energy, the fluid speed behind the turbine would drop to zero. However, if the fluid was not moving behind the turbine, no new fluid would pass through the harvest plane and the turbine would stop. In order to keep fluid moving through the turbine harvest plane there has to be some fluid movement behind the turbine, which creates a limit on the efficiency of a horizontal axis fluid turbine.
The power output available from a horizontal axis turbine can be calculated and is directly proportional to a power coefficient relating to how efficiently a turbine converts the kinetic energy in the fluid stream to mechanical energy and then to electricity.
German physicist Albert Betz calculated that no wind turbine could convert more than 59.3% of the available kinetic energy into mechanical energy as the balance of the available kinetic energy is needed to keep the wind moving through the turbine. This became known as the “Betz Limit” which holds that the absolute maximum power coefficient for a horizontal axis turbine is 0.593. Current conventional technology horizontal axis wind turbines achieve power coefficients that are considerably lower than the Betz Limit.
The Betz Limit is inherent to all currently used horizontal axis wind turbines because of the nature of their operation. Current technology wind turbines operate at a fixed location such that the foils are restricted to harvesting kinetic energy from the ‘stream-tube’ immediately in front of the foils.
Other inherent factors which directly contribute to the power output of conventional horizontal axis fluid turbines include: the area of their harvest plane (more commonly referred to as the ‘swept area’); the total area of the foil/s employed; the tip speed of the foils; the fluid speed; and, the aspect ratio of the foil. The length of currently employed wind turbine air foils are inherently limited due the extreme lever forces acting on the blade root mounting structure.
Other forms of renewable energy generation include various forms of hydro generation, such as dams, tidal flows and small scale run-of-river. All have limitations and some have considerable environmental impacts. Large scale hydroelectric installations have significant environmental impacts due to the need to flood large areas of otherwise arable lands. Further, initial building costs of hydroelectric installations can run into the low billions of dollars per site, with an overall time span from initial site assessment and planning, through to first electricity production that can be more than a decade.
The need for viable, reliable, low cost, clean energy sources is growing. There is a need for a new clean and sustainable energy source with low initial cost infrastructure, with low direct operating costs, with fast construction time frames, that is installable in a wide variety of locations, that has few operating limitations, and that creates few, if any, short or long-term hazards to people, wildlife or the planet as a whole.